The overall objective of this proposal is to develop and to investigate a system for studying the regulation of gene expression in mammalian liver cells. Variant cells have been isolated from the Novikoff hepatoma line that can utilize a pentose (ribose, xylose or arabinose) as a sole carbon and energy source. From these pentose-utilizing cells pentose negative variants can be isolated by BUdR light treatment. Both the pentose positive and pentose negative variants will be studied to determine the genetics and biochemistry of pentose metabolism. Pentose catabolic enzymes will be assayed in the variants growing on glucose and pentose media to determine both the functional pathway(s) for catabolism and the influence of growth media on enzyme levels. Variants unable to use pentoses for growth will be analyzed for the catabolic enzymes to characterize the specific biochemical defect. The negative variants will be used in cellular hybridization experiments to define the number of complementation groups involved in pentose metabolism. Hybridization will also be used to obtain information on the regulation of the enzymes in liver cells.